Raman amplifier, optical repeater, and Raman amplification method

ABSTRACT

A Raman amplifier using semiconductor lasers of Fabry-Perot, DFB, or DBR type or MOPAs, to output pumping lights having different central wavelengths, an interval between adjacent central wavelengths greater than 6 nm and smaller than 35 nm. An optical repeater is adapted to compensate loss in an optical fiber transmission line by the Raman amplifier. A Raman amplification method wherein the shorter the central wavelength of the pumping light, the higher light power of the pumping light. In the Raman amplifier, a certain pumping 1 wavelength being a first channel, and second to n-th channels are arranged with an interval of about 1 THz toward a longer wavelength side, pumping lights having wavelengths corresponding to the first to n-th channels are multiplexed, and pumping light having a wavelength spaced apart from the n-th channel by 2 THz or more toward the longer wavelength side is combined with the multiplexed light, thereby forming the pumping light source.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 13/023,301,filed Feb. 8, 2011, which is a continuation of application Ser. No.12/619,455, filed Nov. 16, 2009, which is a divisional of applicationSer. No. 11/689,352, filed Mar. 21, 2007, now U.S. Pat. No. 7,692,852,which is a divisional of application Ser. No. 10/824,402, filed Apr. 15,2004, now U.S. Pat. No. 7,548,368; which is a divisional of Ser. No.10/120,173, filed Apr. 11, 2002, now U.S. Pat. No. 6,775,057; which is acontinuation of application Ser. No. 09/886,212 filed Jun. 22, 2001, nowU.S. Pat. No. 6,654,162; which is a continuation of application Ser. No.09/527,748 filed Mar. 17, 2000, now U.S. Pat. No. 6,292,288; which is aContinuation-in-Part of PCT/JP99/03944 filed Jul. 23, 1999. The contentsof the above-identified applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a Raman amplifier capable of being usedfor amplification of optical signal in various optical communicationsystems, and optical repeater and Raman amplification method using sucha Raman amplifier, and more particularly, it relates to Raman amplifier,optical repeater and Raman amplification method suitable foramplification of wavelength division multiplexing signals.

2. Related Background Art

Almost all of optical amplifiers used with present optical fibercommunication systems are rare earth doped fiber amplifiers.

Particularly, erbium doped optical fiber amplifier (referred to as“EDFA” hereinafter) using Er (erbium) doped fibers have been usedfrequently. However, a practical gain wavelength band of the EDFA has arange from about 1530 nm to 1610 nm (refer to “Electron. Lett,” vol. 33,no. 23, pp. 1967-1968). Further, the EDFA includes gain havingwavelength dependency, and, thus, when it is used with wavelengthdivision multiplexing signals, difference in gain is generated independence upon a wavelength of optical signal. FIG. 23 shows an exampleof gain wavelength dependency of the EDFA. Particularly, in wavelengthbands smaller than 1540 nm and greater than 1560 nm, change in gainregarding the wavelength is great. Accordingly, in order to obtain givengain (in almost cases, gain deviation is within 1 dB) in the entire bandincluding such wavelength, a gain flattening filter is used.

The gain flattening filter is a filter designed so that loss isincreased in a wavelength having great gain, and the loss profile has ashape substantially the same as that of the gain profile. However, Asshown in FIG. 24, in the EDFA, when magnitude of average gain ischanged, since the gain profile is also changed as shown by curves a, band c, in this case, the optimum loss profile of the gain flatteningfilter is also changed. Accordingly, when the flattening is realized bya gain flattening filter having fixed loss profile, if the gain of theEDFA is changed, the flatness will be worsened.

On the other hand, among optical amplifiers, there is an amplifierreferred to as a Raman amplifier utilizing Raman scattering of anoptical fiber (refer to “Nonlinear Fiber Optics, Academic Press). TheRaman amplifier has peak gain in frequency smaller than frequency ofpumping light by about 13 THz. In the following description, it isassumed that pumping light having 1400 nm band is used, and thefrequency smaller by about 13 THz will be expressed as wavelength longerby about 100 nm. FIG. 25 shows wavelength dependency of gain whenpumping light having central wavelength of 1450 nm is used. In thiscase, peak of gain is 1550 nm and a band width within gain deviation of1 dB is about 20 nm. Since the Raman amplifier can amplify anywavelength so long as an pumping light source can be prepared,application of the Raman amplifier to a wavelength band which could notbe amplified by the EDFA has mainly be investigated. On the other hand,the Raman amplifier has not been used in the gain band of the EDFA,since the Raman amplifier requires greater pump power in order to obtainthe same gain as that of the EDFA. When the pumping light having greatpower is input to a fiber to increase the gain, stimulated Brillouinscattering may be generated. Increase amplification noise caused by thestimulated Brillouin scattering is one of the problems which makes itdifficult to use the Raman amplifier. the Japanese Patent Laid-open No.2-12986 (1990) discloses an example of a technique for suppressing thestimulated Brillouin scattering in the Raman amplifier.

Further, the Raman amplifier has polarization dependency of gain andamplifies only a component (among polarized wave components) coincidedwith the polarized wave of the pumping light. Accordingly, it isrequired for reducing unstability of gain due to polarizationdependency, and, to this end, it is considered that a polarizationmaintaining fiber is used as a fiber for amplifying or an pumping lightsource having random polarization condition.

Furthermore, enlargement of the gain band is required in the Ramanamplifier. To this end, Japanese Patent Publication No. 7-99787 (1995)teaches in FIG. 4 that the pumping light is multiplexed with appropriatewavelength interval. However, this patent does not disclose concretevalues of the wavelength interval. According to a document (K. Rottwitt,OFC98, PD-6), a Raman amplifier using a plurality of pumping lightshaving different wavelengths was reported; however, attempt in theviewpoint of the fact that the gain deviation is reduced below 1 dB wasnot considered.

On the other hand, there is an optical repeater for simultaneouslycompensating for transmission loss and chromatic dispersion in anoptical fiber transmission line, which optical repeater is constitutedby combination of an Er doped fiber amplifier (EDFA) and a dispersioncompensating fiber (DCF). FIG. 46 shows a conventional example in whicha dispersion compensating fiber A is located between two Er doped fiberamplifiers B and C. The first Er doped fiber amplifier B serves toamplify optical signal having low level to a relatively high level andhas excellent noise property. The second Er doped fiber amplifier Cserves to amplify the optical signal attenuated in the dispersioncompensating fiber A to the high level again and has a high outputlevel.

By the way, on designing the optical repeater, it is required that arepeater input level, a repeater output level and a dispersioncompensating amount (loss in the dispersion compensating fiber A) be setproperly, and, there is limitation that the input level of thedispersion compensating fiber A has an upper limit, because, when theinput power to the dispersion compensating fiber A is increased,influence of non-linear effect in the dispersion compensating fiber A isalso increased, thereby deteriorating the transmission wave formconsiderably. The upper limit value of the input power to the dispersioncompensating fiber A is determined by self phase modulation (SPM) in onewave transmission and by cross phase modulation (XPM) in WDMtransmission. Thus, regarding the optical repeater, an optical repeaterhaving excellent gain flatness and noise property must be designed inconsideration of the several variable factors.

FIG. 47 shows a signal level diagram in the repeater. Gain G1. [dB] ofthe first Er doped fiber amplifier B is set to a difference between aninput level Pin [dB] of the repeater and an input upper limit value Pd[dB] to the dispersion compensating fiber A. Gain G2 [dB] of the secondEr doped fiber amplifier C is set to (Gr+Ld−G1) [dB] from loss Ld [dB]in the dispersion compensating fiber A, gain Gr [dB] of the repeater andthe gain G1 [dB] of the first Er doped fiber amplifier B. Since thesedesign parameters are varied for each system, the values G1 [dB] and G2[dB] are varied for each system, and, accordingly, the Er doped fiberamplifiers B, C must be re-designed for each system. The noise propertyin such a system is deeply associated with the loss Ld [dB] in thedispersion compensating fiber A, and it is known that the greater theloss the more the noise property is worsened. Further, at present, a gapfrom the designed value in loss in the transmission line and the loss inthe dispersion compensating fiber A are offset by changing the gains ofEr doped fiber amplifiers B, C. In this method, the gain of Er dopedfiber amplifier B and C are off the designed value, thus the gainflatness is worsened. A variable attenuator may be used to offset thegap from the designed value of loss. In this method, although the gainflatness is not changed, an additional insertion loss worsens the noiseproperty.

In the optical fiber communication system, although the Er doped opticalfiber amplifiers have widely been used, the Er doped optical fiberamplifier also arises several problems. Further, the Raman amplifieralso has problems that, since output of ordinary semiconductor laser isabout 100 to 200 mW, gain obtained is relatively small, and that thegain is sensitive to change in power or wavelength of the pumping light.So that, when a semiconductor laser of Fabry-Perot type havingrelatively high output is used, noise due to gain fluctuation caused byits mode hopping becomes noticeable, and that, when the magnitude of thegain is adjusted, although drive current of the pumping laser must bechanged, if the drive current is changed, since the fluctuation in thecentral wavelength is about 15 nm at the maximum, the wavelengthdependency of gain will be greatly changed. Further, such shifting ofthe central wavelength is not preferable because such shifting causeschange in joining loss of a WDM coupler for multiplexing the pumpinglight. In addition, the optical repeater also has a problem that the Erdoped optical fiber amplifiers B, C must be re-designed for each system.Further, the deterioration of the noise property due to insertion of thedispersion compensating fiber is hard to be eliminated in the presentsystems.

In a Raman amplification method for amplifying optical signal by using astimulated Raman scattering phenomenon, a communication optical fiber isused as an optical fiber acting as an amplifying medium, and, in adistributed amplifying system, a wavelength of pumping light and awavelength of the optical signal are arranged in 1400 nm-1600 nm bandhaving low loss and low wavelength dependency within a wide band of thecommunication optical fiber. In this case, regarding the loss ofwavelength dependency of the optical fiber as the amplifying medium, adifference between maximum and minimum values is below about 0.2 dB/kmin the above band, even in consideration of loss caused by hydroxyl ion(OH) having peak at 1380 nm. Further, even if each pumping powers in amulti-wavelength pumping system are not differentiated according to thewavelength dependence of the loss, the gain of the signals amplified bythe pumping lights are substantially the same, there is no problem inpractical use.

On the other hand, in a Raman amplifier operating as a discreteamplifier such as EDFA (rare earth doped fiber amplifier) it isnecessary to pay attention to the package of the amplifier fiber, for alength of the fiber is about 10 km to about several tens of kilometersin order to obtain the required gain. Thus, it is convenient that thelength of the fiber is minimized as less as possible. Although thelength of the fiber can be shortened by using an optical fiber havinggreat non-linearity, in the optical fiber having great non-linearity, itis difficult to reduce the transmission loss caused by (OH) generallyhaving a band of 1380 nm, and Rayleigh scattering coefficient becomesgreat considerably in comparison with the communication fiber, with theresult that the difference between the maximum and minimum values offiber loss within the above-mentioned wavelength range becomes verygreat such as 1.5 to 10 dB/km. This means that, when the optical fiberhaving length of 3 km is used, the loss difference due to the wavelengthof the pumping light becomes 4.5 dB to 30 dB. Thus, the wavelengthdivision multiplexing signals cannot be uniformly amplified by using thepumping lights having the same intensities.

As one of means for multiplexing the number of pumping lights, there isa wavelength combiner of Mach-Zehnder interferometer type. Since theMach-Zehnder interferometer has periodical response property regardingfrequency, the wavelength of the pumping light must be selected amongwavelengths having equal intervals in frequency. Accordingly, thewavelength combiner of Mach-Zehnder interferometer type has limitationin degrees of freedom of wavelength setting, but has an advantage that,when a device of waveguide type or fiber fusion type, if the number ofwavelength division multiplexing is increased, insertion loss is notchanged substantially.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a Raman amplificationmethod capable of uniformly amplifying wavelength division multiplexingsignals and suitable to be incorporated as a unit.

Another object of the present invention is to provide a Raman amplifierwhich can obtain required gain and can reduce wavelength dependency ofgain to the extent that usage of a gain flattening filter is notrequired and which can be used in a band of EDFA.

A further object of the present invention is to apply the Ramanamplifier to an optical repeater constituted by an Er doped fiberamplifier (EDFA) and a dispersion compensating fiber (DCF) thereby toprovide an optical repeater in which the EDFA is not required to bere-designed for each system and which can compensate dispersion intransmission line loss and/or DCF loss without deteriorating property ofthe optical repeater.

Further, by Raman-amplifying the DCF, the deterioration of noiseproperty due to insertion of the DCF which could not avoided in theconventional techniques is reduced.

An example of a Raman amplifier according to the present invention isshown in FIGS. 1, 2 and 3. When a small-sized semiconductor laser 3 ofFabry-Perot type having relatively high output is used in a pumpingmeans 1, relatively high gain can be obtained, and, since thesemiconductor laser 3 of Fabry-Perot type has a wide line width of anoscillating wavelength, occurrence of stimulated Brillouin scatteringdue to the pumping light can be eliminated substantially. On the otherhand, when a semiconductor laser of DFB type or DBR type, or a MaterOscillator Power Amplifier (MOPA) is used in the pumping means 1, sincea fluctuation range of the oscillating wavelength is relatively small, again configuration is not changed by a driving condition. Further,occurrence of stimulated Brillouin scattering can be suppressed byeffecting modulation.

Further, by selecting the interval between the central wavelengths ofthe pumping light to a value greater than 6 nm and smaller than 35 nm,the wavelength dependency of gain can be reduced to the extent that thegain flattening filter is not required. The central wavelength λc inthis case is a value defined regarding the single pumping light and isrepresented by the following equation when it is assumed that awavelength of an i-th longitudinal mode of laser oscillating is λi andlight power included in that mode is Pi:

The reasons why the interval between the central wavelengths of thepumping light is selected to a value greater than 6 nm is that theoscillating band width of the semiconductor laser 3 of Fabry-Perot typeconnected to an external resonator 5 having narrow reflection band widthis about 3 nm as shown in FIG. 12, and that a WDM coupler 11 (FIGS. 1, 2and 3) for combining the pumping lights is permitted to have certainplay or margin in wavelength interval between the pumping lights inorder to improve wave combining efficiency. The WDM coupler 11 isdesigned so that lights having different wavelengths are received bydifferent ports and the incident lights are joined at a single outputport substantially without occurring of loss of lights. However,regarding light having intermediate wavelength between the designedwavelengths, the loss is increased even whichever port is used. Forexample, in a certain WDM coupler 11, a width of a wavelength band whichincreases the loss is 3 nm. Accordingly, in order that the band of thesemiconductor laser 3 is not included within said band, as shown in FIG.12, a value 6 nm obtained by adding 3 nm to the band width of thesemiconductor laser 3 is proper for low limit of the interval betweenthe central wavelengths of the pumping light. On the other hand, asshown in FIG. 13A, if the interval between the central wavelengths ofthe semiconductor laser 3 is greater than 35 nm, as shown in FIG. 13B, again valley is created at an intermediate portion of the Raman gain bandobtained by the pumping lights having adjacent wavelengths, therebyworsening the gain flatness. The reason is that, regarding the Ramangain obtained by the single pumping light, at a position spaced apartfrom the gain peak wavelength by 15 nm to 20 nm, the gain is reduced tohalf. Accordingly, by selecting the interval of the central wavelengthsof the pumping light to the value greater than 6 nm and smaller than 35nm, the wavelength dependency of gain can be reduced to the extent thatthe gain flattening filter is not required.

According to a second aspect of the present invention, since a Ramanamplifier has a control means 4 for monitoring input light or outputlight with respect to the Raman amplifier and for controlling pumppowers of the pumping means 1 on the basis of a monitored result tomaintain output light power of the Raman amplifier to a predeterminedvalue, given output can be obtained regardless of fluctuation of inputsignal power to the Raman amplifier and/or dispersion in loss of a Ramanamplifier fiber.

According to a third aspect of the present invention, since a Ramanamplifier has a controlling means 4 for flattening Raman gain, the gaincan be flattened. Particularly, as shown in FIG. 16, by monitoringlights having wavelengths obtained by adding about 100 nm to thewavelengths of the pumping lights, respectively, and by controlling thepowers of the pumping lights to order or align the monitored lightpowers, the gain can be flattened. Further, since a wavelengthstabilizing fiber grating (external resonator 5) which will be describedlater can suppress the shift in the central wavelength due to change indrive current of a Fabry-Perot type semiconductor laser, it can also beused as a means for permitting control of the gain.

According to a fourth aspect of the present invention, since a Ramanamplifier has a control means 49 for monitoring input signal power andoutput signal power and for controlling pumping light power to make aratio between the input signal power and the output signal powerconstant thereby to maintain gain of the Raman amplifier to apredetermined value, given output can be obtained regardless offluctuation of the input signal power to the Raman amplifier and/ordispersion in loss of a Raman amplifier fiber.

According to a fifth aspect of the present invention, in a Ramanamplifier, since an optical fiber having non-linear index n2 ofrefraction of 3.5E-20 [m2/W] or more is used as an optical fiber 2,adequate amplifying effect can be obtained, from the result ofinvestigation.

According to a sixth aspect of the present invention, in a Ramanamplifier, since the optical fiber 2 exists as a part of a transmissionfiber for propagating the optical signal, the amplifier can beincorporated into the transmission optical fiber as it is.

According to a seventh aspect of the present invention, a Ramanamplifier utilizes a part of a dispersion managed transmission line andcan constitute an amplifier as it is as a amplifying medium.

According to an eighth aspect of the present invention, in a Ramanamplifier, since the optical fiber 2 exists as an amplifier fiberprovided independently from a transmission fiber for propagating theoptical signal and inserted into the transmission fiber, an opticalfiber suitable for Raman amplification can easily be used for theoptical fiber 2 and the chromatic dispersion compensating fiber caneasily be utilized, and a discrete amplifier can be constituted.

In an optical repeater according to the present invention, since loss ofan optical fiber transmission line 8 is compensated by using the Ramanamplifier, an optical repeater having the function of the Ramanamplifier can be provided.

Among optical repeaters of the present invention, in a repeater in whichrare earth doped fiber amplifier(s) 10 is (are) provided at preceding orfollowing stage or at both stages of the Raman amplifier, since the lossof the optical fiber transmission line 8 is compensated by the Ramanamplifier 9 and the rare earth doped fiber amplifier(s) 10, desiredamplifying property suitable for various transmission systems can beobtained.

Among optical repeaters of the present invention, in accordance with anarrangement in which the Raman amplifier 9 and the rare earth dopedfiber amplifier 10 are combined, a repeater adapted to various systemscan be obtained. This fact will be explained hereinbelow as an examplethat DCF is used as the amplifier fiber of the Raman amplifier 9. FIG.17 shows design parameters of a conventional optical repeater, and thegains G1, G2 are varied for each system. Further, it is not inevitablethat input of the repeater and loss of the DCF are fluctuated byscattering in repeater spacing and scattering in the DCF. Suchfluctuation is directly associated with the fluctuation of the gain ofthe EDFA, which fluctuation of the gain leads to deterioration offlatness. FIG. 18 schematically shows a relationship between theflatness and the gain of the EDFA. Since the flatness is optimized bylimiting the used band and the average gain, if the average gain isdeviated from the optimum point, the wavelength dependency of gain ischanged to worsen the flatness. In order to avoid the deterioration ofthe flatness, the gain of the EDFA must be kept constant.Conventionally, a variable attenuator has been used as a means forcompensating the fluctuation in input level and loss of the DCF. FIG.19A shows an example that an attenuating amount of the variableattenuator is adjusted in accordance with the fluctuation of the inputlevel to control the input level to the DCF to be kept constant and FIG.19B shows an example that an attenuating amount is adjusted inaccordance with the fluctuation in loss of the DCF to controlintermediate loss to be kept constant. In both examples, the gains oftwo amplifiers are constant. However, in these methods, since uselessloss is added by the variable attenuator, there is an disadvantage inthe viewpoint of noise property. In the present invention, bycompensating the change in design parameters of the repeater by theRaman amplification of the DCF, the gain of the EDFA is kept constant,requirement that the EDFA be re-designed for each system is eliminated,and the scattering in repeater spacing and scattering in the DCF can becompensated without sacrificing the flatness and the noise property.FIG. 20 shows design values of the EDFA when the Raman amplification ofthe DCF is applied to the specification of the repeater of FIG. 17. Byselecting the Raman gain of the DCF appropriately, the properties of theEDFA required for three specifications can be made common. Further, asshown in FIGS. 21A and 21B, the fluctuation in input level and in lossof the DCF can be compensated by changing the Raman gain withoutchanging the gain of the EDFA. In any cases, the Raman gain is adjustedso that the output level of the DCF becomes constant, while keeping thegain of the EDFA constant. Further, by compensating the loss of the DCFitself by the Raman amplification, the deterioration of the noiseproperty due to insertion of the DCF which could not avoided in theconventional techniques can be reduced. FIG. 37 shows measured values ofa deteriorating amount of the noise figure when the DCF is inserted andof a deteriorating amount of the noise figure when the Raman amplifierusing the same DCF is inserted.

In the optical repeater according to the present invention, when a Ramanamplifier using an pumping light source in which wavelengths are notcombined is provided, although an operating wavelength range isnarrower, a construction can be simplified and the same property as theaforementioned optical repeaters can be obtained, except for the bandwidth, in comparison with an optical repeater having a Raman amplifierpumped by a plurality of wavelengths. FIGS. 38 and 39 show measuredexamples of the optical repeater using the Raman amplifier pumped by thepumping light source in which wavelengths are not combined and of theoptical repeater using the Raman amplifier pumped by the plurality ofwavelengths.

In the Raman amplifier according to the present invention, when adifference between maximum and minimum values of the central wavelengthof the pumping light is within 100 nm, overlapping between the pumpinglight and the optical signal can be prevented to prevent wave formdistortion of the optical signal. If the wavelength of the pumping lightis similar to the wavelength of the optical signal, since the wave formof the optical signal may be deteriorated, the wavelength of the pumpinglight and the wavelength of the optical signal must be selected so thatthey are not overlapped. However, in a case where the pumping light hasband of 1.4 μm, when the difference between the maximum and minimumvalues of the central wavelength of the pumping light is smaller than100 nm, as shown in FIG. 14, since the difference between the centralwavelength of the gain caused by one pumping light and the wavelength ofsuch pumping light is about 100 nm, the overlapping between thewavelength of the pumping light and the wavelength of the optical signalcan be prevented.

In the Raman amplifier according to the present invention, when thepumping lights having adjacent wavelengths are propagated through theoptical fiber 2 toward two different directions so that the opticalsignal pumped bi-directionally, the wavelength property required in theWDM coupler 11 shown in FIG. 1 and FIGS. 2 and 3 can be softened. Thereason is that, as shown in FIG. 15, in all of pumping lights from bothdirections, although the central wavelengths become λ₁, λ₂, λ₃, λ₄ andthe interval is greater than 6 nm and smaller than 35 nm, whenconsidering the pumping lights from only one direction, the centralwavelengths become λ₁ and λ₃ or λ₂ and λ₄ and the wavelength intervalincreased to twice, with the result that the property required in theWDM coupler 11 can have margin or play.

In the Raman amplifier according to the present invention, when thewavelength stabilizing external resonator 5 such as fiber grating isprovided at the output side of the semiconductor laser 3 of Fabry-Perottype, noise due to fluctuation of caused by mode hopping of thesemiconductor laser 3 of Fabry-Perot type can be suppressed. Inconsideration of one pumping light source, it makes the bandwidthnarrower to connect the wavelength stabilizing external resonator 5 tothe semiconductor laser 3. Since it also results the smaller wavelengthinterval in case of combining pumping light sources by the WDM coupler11 (FIGS. 1, 2 and 3), pumping light having higher output and wider bandcan be generated.

In the Raman amplifier according to the present invention, when thepumping light of the semiconductor laser 3 is used to bepolarization-combined for each wavelength, not only polarizationdependency of gain can be eliminated but also the pump power launchedinto the optical fiber 2 can be increased. In the Raman amplification,only the components matched with the polarized of the pumping light canbe given the gain. When the pumping light is linear-polarized and theamplifier fiber is not a polarization maintaining fiber, the gain ischanged due to fluctuation of relative state of polarization of thesignal and the pumping light. Therefore, the polarization dependence ofgain can be eliminated by combining two linear-polarized pumping lightsso that the polarization planes are perpendicular to each other.Further, it increases the pumping light power launched into the fiber.

In the Raman amplifier according to the present invention, in the casewhere a wavelength combiner of planar lightwave circuit based on aMach-Zehnder interferometer is used as a means for wave-combining MOPAor a semiconductor laser of Fabry-Perot type, DFB type or DBR typehaving a plurality of wavelengths, even when the number of thewavelengths to be combined is large, the wave-combination can beachieved with very low loss, and pumping light having high output can beobtained.

In the Raman amplifier according to the present invention, as shown inFIG. 6, when a polarization plane rotating means 7 for rotating thepolarization plane by 90 degrees is provided so that the optical fiber 2simultaneously includes the plurality of pumping lights generated by thepumping means 1 and the pumping lights having a orthogonal state ofpolarization to the pumping lights generated by the pumping means 1, inprinciple, given gain can always be obtained regardless of the state ofpolarization of the optical signal, with the result that the band of theRaman gain can be widened.

In the optical repeater according to the present invention, when theRaman amplification is utilized by coupling residual pumping light ofthe Raman amplifier to the optical fiber transmission line 8, a part ofloss of the optical fiber transmission line 8 can be compensated.

In the optical repeater according to the present invention, when theresidual pumping light of the Raman amplifier is utilized as pumpinglight for the rare earth doped fiber amplifier 10, the number of thesemiconductor lasers to be used can be reduced.

In the optical repeater according to the present invention, when adispersion compensating fiber is used as the optical fiber 2 of theRaman amplifier 9, the chromatic dispersion of the optical fibertransmission line 8 can be compensated by the Raman amplifier 9, and apart or all of the losses in the optical fiber transmission line 8 andthe amplifier fiber 2 can be compensated.

According to a twenty-ninth aspect of the present invention, in a Ramanamplification method by stimulated Raman scattering in an optical fiberthrough which two or more pumping lights having different centralwavelengths and said optical signals are propagated, the pumping powerlaunched into said optical fiber increases as the central wavelengths ofsaid pumping lights is shorter.

According to a thirtieth aspect of the present invention, in a Ramanamplification method by stimulated Raman scattering in an optical fiberthrough which two or more pumping lights having different centralwavelengths and said optical signals are propagated, total pumping poweron the shorter wavelength side with respect to the center between theshortest and longest central wavelengths among said two or more pumpinglights is greater than on the longer side.

According to a thirty-first aspect of the present invention, in a Ramanamplification method by stimulated Raman scattering in an optical fiberthrough which three or more pumping lights having different centralwavelengths and said optical signals are propagated, the number of thepumping light sources on the shorter wavelength side with respect to thecenter between the shortest and longest central wavelengths among saidthree or more pumping lights is greater than on the longer side, and thetotal pumping power on the shorter wavelength side is greater than onthe longer side.

To achieve the above object, according to thirty-second to thirty-fourthaspects of the present invention, when the shortest pumping wavelengthis defined as a first channel and, from the first channel, at respectiveintervals of about 1 THz toward the longer wavelength, second to n-thchannels are defined, pumping lights having wavelengths corresponding tothe first to n-th channels are multiplexed and pumping light having awavelength spaced apart from the n-th channel by 2 THz or more towardthe longer wavelength is combined with the multiplexed light, and lightobtained in this way is used as the pumping light of the Ramanamplifier. When the shortest pumping wavelength is defined as the firstchannel and, from the first channel, at respective intervals of about 1THz toward the longer wavelength, the second to n-th channels aredefined, light obtained by combining all of the wavelengthscorresponding to the channels other than (n−1)-th and (n−2)-th channelsis used as the pumping light of the Raman amplifier. Alternatively,light obtained by combining all of the wavelengths corresponding to thechannels other than (n−2)-th and (n−3)-th channels is used as thepumping light of the Raman amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a first embodiment of a Raman amplifieraccording to the present invention;

FIG. 2 is a diagram showing a second embodiment of a Raman amplifieraccording to the present invention;

FIG. 3 is a diagram showing a third embodiment of a Raman amplifieraccording to the present invention;

FIG. 4 is a diagram showing a first example of a controlling means inthe Raman amplifier according to the present invention;

FIG. 5 is a diagram showing a second example of a controlling means inthe Raman amplifier according to the present invention;

FIGS. 6A and 6B are diagrams showing different examples of apolarization plane rotating means in the Raman amplifier according tothe present invention;

FIG. 7 is a diagram showing a first embodiment of an optical repeateraccording to the present invention;

FIG. 8 is a diagram showing a second embodiment of an optical repeateraccording to the present invention;

FIG. 9 is a diagram showing a third embodiment of an optical repeateraccording to the present invention;

FIG. 10 is a diagram showing a fourth embodiment of an optical repeateraccording to the present invention;

FIG. 11 is a diagram showing a fifth embodiment of an optical repeateraccording to the present invention;

FIG. 12 is an illustration of why a wavelength interval of pumping lightis selected to be greater than 6 nm;

FIGS. 13A and 13B are illustrations of why a wavelength interval ofpumping light is selected to be smaller than 35 nm;

FIG. 14 is an illustration of why a difference between a longestwavelength and a shortest wavelength is selected to be smaller than 100nm;

FIG. 15 is an illustration of an example of wavelength arrangement ofpumping lights in bi-directional pumping;

FIG. 16 is an illustration of how a condition that a gain over aspecified band is flattened by controlling pumping light power;

FIGS. 17A and 17B are illustrations of properties associated withdesigning of the optical repeater;

FIG. 18 is an illustration of a relationship between fluctuation in gainof EDFA and deterioration of flatness;

FIG. 19A is an illustration of how a condition that fluctuation in inputlevel due to a variable attenuator is compensated, and FIG. 19B is anexplanatory view showing a condition that fluctuation in loss of DCF dueto a variable attenuator is compensated;

FIGS. 20A and 20B are illustrations of properties associated withdesigning of the optical repeater utilizing a Raman amplification inDCF;

FIG. 21A is an illustration of how a condition that fluctuation in inputlevel due to the Raman amplification is compensated, and FIG. 21B is anexplanatory view showing a condition that fluctuation in loss of DCF dueto the Raman amplification is compensated;

FIG. 22 is an illustration of different examples of output spectrum inthe Raman amplifier;

FIG. 23 is an illustration of wavelength dependency of gain due to EDFA;

FIG. 24 is an illustration of fluctuation in gain due to EDFA;

FIG. 25 is an illustration of wavelength dependency of gain due to Ramanamplification;

FIG. 26 is a diagram showing a control method for controlling outputlight power by monitoring input light;

FIG. 27 is a diagram showing a control method for controlling outputlight power by monitoring output light;

FIG. 28 is a diagram showing a control method for controlling outputlight power by monitoring input light and output light;

FIG. 29 is a diagram showing a first example of a method for obtainingRaman gain by coupling residual pumping light of the Raman amplifier toan optical fiber transmission line;

FIG. 30 is a diagram showing a second example of a method for obtainingRaman gain by coupling residual pumping light of the Raman amplifier toan optical fiber transmission line;

FIG. 31 is a diagram showing a third example of a method for obtainingRaman gain by coupling residual pumping light of the Raman amplifier toan optical fiber transmission line;

FIG. 32 is a diagram showing a fourth example of a method for obtainingRaman gain by coupling residual pumping light of the Raman amplifier toan optical fiber transmission line;

FIG. 33 is a diagram showing a first example of a method for utilizingthe residual pumping light of the Raman amplifier as pumping light ofEDFA;

FIG. 34 is a diagram showing a second example of a method for utilizingthe residual pumping light of the Raman amplifier as pumping light ofEDFA;

FIG. 35 is a diagram showing a third example of a method for utilizingthe residual pumping light of the Raman amplifier as pumping light ofEDFA;

FIG. 36 is a diagram showing a fourth example of a method for utilizingthe residual pumping light of the Raman amplifier as pumping light ofEDFA;

FIG. 37 is an illustration of deterioration of noise figure due toinsertion of a dispersion compensating fiber;

FIG. 38 is an illustration of the number of pumping wavelengths of theRaman amplifier and property of the repeater;

FIG. 39 is an illustration of the number of pumping wavelengths of theRaman amplifier and property of the repeater;

FIG. 40 is a diagram of an optical repeater in which a plurality ofRaman amplifiers are connected in a multi-stage fashion;

FIG. 41 is a diagram showing an example of a pumping means having asingle pumping light source;

FIG. 42 is a diagram showing another example of a pumping means having asingle pumping light source;

FIG. 43 is a diagram showing an example of a pumping means having twopumping light sources;

FIG. 44 is a diagram showing another example of a pumping means havingtwo pumping light sources;

FIG. 45 is a diagram of a Raman amplifier in which a dispersioncompensating fiber is used as an amplifier fiber;

FIG. 46 is a diagram showing an example of a conventional opticalrepeater;

FIG. 47 is an illustration of signal level diagram in the opticalrepeater of FIG. 46;

FIG. 48 is a diagram showing a case where SMF and a fiber havingdispersion smaller than −20/ps/nm/km are used as the amplifier fiber;

FIG. 49 is an illustration of an example of an arrangement for carryingout a Raman amplification method according to the present invention;

FIG. 50 is an explanatory view showing an pumping light source of FIG.49;

FIG. 51 is a view showing gain profile of a optical signalRaman-amplified by the Raman amplifying method according to the presentinvention;

FIG. 52 is a view showing gain profile of a optical signalRaman-amplified by a conventional method;

FIG. 53 is an explanatory view showing another example of an arrangementfor carrying out a Raman amplification method according to the presentinvention;

FIGS. 54A and 54B are views showing Raman gain profiles when an intervalbetween pumping light is selected to 4.5 THz and 5 THz, respectively andDSF is used as the amplifier fiber;

FIG. 55 is a view showing Raman gain profile when the interval betweenpumping light is selected to 4.5 THz and three wavelengths are used;

FIG. 56 is a view showing Raman gain profile when the interval betweenpumping light is selected to 2.5 THz and 4.5 THz and three wavelengthsare used;

FIG. 57 is a view showing performance of Raman gain curves when theintervals of the pumping lights are equidistant and the peak gains areadjusted to 10 dB on the same;

FIG. 58 is a view showing performance of Raman gain curves when theintervals of the pumping lights are equidistant and the gains generatedby the respective pumping lights are adjusted so that the gain curvesare flattened;

FIG. 59 is a view showing performance of Raman gain curves when theintervals of the pumping lights are equidistant of 1 THz and the numberof multiplexing is changed;

FIG. 60 is a schematic diagram of a sixth embodiment of a Ramanamplifier according to the present invention;

FIG. 61 is a view showing gain profile when a pumping light source ofFIG. 60 is used;

FIG. 62 is an enlarged view of total gain shown in FIG. 61;

FIG. 63 is a view showing Raman gain profile when a wavelength of asixth channel is a wavelength spaced apart from a fifth channel by 2.5THz toward the longer wavelength in the Raman amplifier shown in FIG.60;

FIG. 64 is an enlarged view of total gain shown in FIG. 63;

FIG. 65 is a schematic diagram of a seventh embodiment of a Ramanamplifier according to the present invention;

FIG. 66 is a view showing Raman gain profile when an pumping lightsource of FIG. 65 is used;

FIG. 67 is an enlarged view of total gain shown in FIG. 66;

FIG. 68 is an explanatory view showing an eighth embodiment of a Ramanamplifier according to the present invention;

FIG. 69 is a view showing Raman gain profile when an pumping lightsource of FIG. 68 is used;

FIG. 70 is an enlarged view of total gain shown in FIG. 69;

FIG. 71 is a view showing Raman gain profile when eleven channels areused among thirteen channels at interval of 1 THz from 211 THz to 199THz and pumping lights other than 201 THz and 200 THz are used;

FIG. 72 is an enlarged view of total gain shown in FIG. 71;

FIG. 73 is a view showing Raman gain profile when eleven channels areused among thirteen channels at interval of 1 THz from 211 THz to 199THz and pumping lights other than 202 THz and 201 THz are used; and

FIG. 74 is an enlarged view of total gain shown in FIG. 73.

BEST MODE FOR CARRYING OUT THE INVENTION

(First Embodiment of Raman Amplifier)

FIG. 1 shows a first embodiment of a Raman amplifier according to thepresent invention. The Raman amplifier comprises a signal input fiber12, an amplifier fiber (optical fiber) 2, a WDM coupler 13, a pumpingmeans 1, a tap coupler for monitoring 14, a monitor signal detecting andLD control signal generating circuit 15, a signal output fiber 16, and apolarization independent isolator 25. The tap coupler for monitoring 14and the monitor signal detecting and LD control signal generatingcircuit 15 constitute a controlling means 4.

The pumping means 1 includes semiconductor lasers 3 (3 ₁, 3 ₂, 3 ₃, 3 ₄)of Fabry-Perot type, wavelength stabilizing fiber ratings (externalresonators) 5 (5 ₁, 5 ₂, 5 ₃, 5 ₄), polarization coupler (polarizationbeam combiner) 6 (6 ₁, 6 ₁), and a WDM coupler 11. Oscillatingwavelengths of the semiconductor lasers 3 ₁, 3 ₂ and permeationwavelengths of the fiber gratings 5 ₁, 5 ₂ are the same wavelength λ₁,and central wavelengths of the semiconductor lasers 3 ₃, 3 ₄ andpermeation wavelengths of the fiber gratings 5 ₃, 5 ₄ are the samewavelength λ₂, and the oscillating wavelengths of the semiconductorlasers 3 ₁, 3 ₂, 3 ₃, 3 ₄ can be subjected to the action of thewavelength stabilizing fiber gratings 5 ₁, 5 ₂, 5 ₃, 5 ₄ so that thecentral wavelengths are stabilized to λ₁, λ₂. Further, a wavelengthinterval between the wavelength λ₁ and the wavelength λ₂ is selected tobe greater than 6 nm and smaller than 35 nm.

Pumping lights generated by the semiconductor lasers 3 ₁, 3 ₂, 3 ₃, 3 ₄are polarization-combined by the polarization coupler 6 for eachwavelength λ₁, λ₂, and output lights from the polarization coupler 6 arecombined by the WDM coupler 11 to obtain output light of the pumpingmeans 1. Polarization maintaining fibers 17 are connected between thesemiconductor lasers 3 and the polarization coupler 6 to obtain twopumping lights having different polarization planes. The output light ofthe pumping means 1 is coupled to the amplifier fiber 2 via the WDMcoupler 13; on the other hand, an optical signal (wavelength divisionmultiplexing signal) is incident on the amplifier fiber 2 from theoptical signal input fiber 12 and then is combined with the pumpinglight of the pumping means 1 in the amplifier fiber 2 to beRaman-amplified, and the Raman-amplified light is passed through the WDMcoupler 13 and is sent to the monitor light branching coupler 14, wherea part of the light is branched as a monitor signal, and the other isoutputted to the optical signal output fiber 16. The monitor signal ismonitored in the monitor signal detecting and LD control signalgenerating circuit 15, and the circuit 15 generate a signal forcontrolling drive currents for the semiconductor lasers 3 so that gaindeviation in the signal wavelength band becomes small.

The amplifier fiber 2 may be a special fiber suitable for Ramanamplification (for example, a fiber having non-linear index ofrefraction n2 of 3.5 E−20 [m2/W] or more) or may be an extension of thesignal input fiber 12 by which the optical signal is received. Further,as shown in FIG. 48, RDF (reverse dispersion fiber) having dispersion ofless than −20 ps/nm per 1 km may be connected to SMF so that theamplifier fiber can also act as a transmission line. (Generally, sinceRDF has dispersion of less than −20 ps/nm, RDF having a lengthsubstantially the same as or greater, by twice, than a length of SMF maybe used.) In such a case, preferably, it is designed so that the Ramanamplifying pumping light is propagated from the RDF toward the SMF. Inthe Raman amplifier, the amplifier fiber 2 may be connected to andinserted into a transmission fiber (not shown) to which the opticalsignal is transmitted, and the amplifier fiber 2, pumping means 1, WDMcoupler 13, monitor light branching coupler 14, and monitor signaldetecting and LD control signal generating circuit 15 may beincorporated to constitute a concentrated Raman amplifier.

FIG. 22 shows the output spectrum of the Raman amplifier of FIG. 1. Inthis experiment, the wavelength of 1435 nm and 1465 nm were used as thepumping wavelengths λ₁ and λ₂ in FIG. 1. The power of the input signalsis −20 dBm/ch, and the wavelengths are arranged at an even space between1540 nm and 1560 nm. The amplifier fiber 2 was a dispersion compensatingfiber having a length of about 6 km, and powers of the pumping lightswere adjusted to compensate the loss of the dispersion compensatingfiber while keeping deviation between channels within 0.5 dB.

(Second Embodiment of Raman Amplifier)

FIG. 2 shows a second embodiment of a Raman amplifier according to thepresent invention, in which the pumping light from the pumping means 1is propagated toward the same direction of the optical signal in theamplifier fiber 2. More specifically, the WDM coupler 13 is provided ata input end of the amplifier fiber 2, and the pumping light from thepumping means 1 is transmitted, through the WDM coupler 13, from theinput end to output end of the amplifier fiber 2. In this arrangement,since amplification is effected before attenuation of signal, the noiseproperty of the optical signal is superior to that in the firstembodiment. Further, it was found that the gain is smaller in comparisonwith the first embodiment.

(Third Embodiment of Raman Amplifier)

FIG. 3 shows a third embodiment of a Raman amplifier according to thepresent invention, in which pumping lights from an pumping means 1 arepropagated in two directions through the amplifier fiber 2. Morespecifically, WDM couplers 13 are provided at input and output ends ofthe amplifier fiber 2, respectively, and the pumping lights from twogroups of the pumping means 1 are coupled to the amplifier fiber 2through the respective WDM couplers 13 so that the pumping lightlaunched into the input side WDM coupler 13 is propagated toward theoutput end of the amplifier fiber 2 and the pumping light launched intothe output side WDM coupler 13 is propagated toward the input end of theamplifier fiber 2.

Central wavelengths of semiconductor lasers 3 ₁, 3 ₂ included in thefirst group A of the pumping means 1 and central wavelengths ofsemiconductor lasers 3 ₅, 3 ₆ included in the second group B are thesame, and central wavelengths of semiconductor lasers 3 ₃, 3 ₄ includedin the first group A and central wavelengths of semiconductor lasers 3₇, 3 ₈ included in the second group B are the same. Further, fibergratings 5 ₁ to 5 ₈ are matched with the central wavelength of thesemiconductor lasers 3 to which the fiber gratings are connected,respectively.

(Fourth Embodiment of Raman Amplifier)

In the embodiment shown in FIG. 3, when it is assumed that the centralwavelengths of the semiconductor lasers 3 ₁, 3 ₂ included in the firstgroup A are λ₁, central wavelengths of the semiconductor lasers 3 ₃, 3 ₄included in the first group A are λ₃, central wavelengths of thesemiconductor lasers 3 ₅, 3 ₆ included in the second group B are λ₂, andcentral wavelengths of the semiconductor lasers 3 ₇, 3 ₈ included in thesecond group B are λ₄, the wavelengths λ₁, λ₂, λ₃, λ₄ may be adjacentwavelengths. Also in this case, the interval between the centralwavelengths is greater than 6 nm and smaller than 35, and the differencebetween the maximum central wavelength λ₄ and the minimum centralwavelength λ₁ is smaller than 100 nm. With this arrangement, thewavelength interval of the pumping lights combined in the same group canhave play or margin and performance required for the WDM couplers 13 canbe loosened.

(Fifth Embodiment of Raman Amplifier)

FIG. 40 shows a fifth embodiment of a Raman amplifier according to thepresent invention. In this embodiment, appropriate Raman amplifiers areselected among the Raman amplifiers 9 described in the above-mentionedembodiments and the selected Raman amplifiers are connected together ina multi-stage fashion. By properly selecting the Raman amplifiers 9having different characteristics in accordance with the desiredamplifying property and noise property, a property which could not beachieved by a single Raman amplifier can be obtained.

In the above-mentioned embodiments, the output light power controllingmeans 4 may be designed as shown in FIG. 4 or FIG. 5. In an arrangementshown in FIG. 4, a monitor signal detecting and LD control signalgenerating circuit 15 comprising a wavelength demultiplexer 18,optical/electrical converting means 19 such as photo-diodes and an LDcontrol circuit 20 is connected to a monitor light branching coupler 14as shown in FIG. 1, 2, or 3. The wavelength demultiplexer 18 serves todemultiplex the output light branched by the monitor light branchingcoupler 14 into a plurality of wavelength lights. In this case, lightsnear the maximum amplification wavelengths (each of which is awavelength obtained by adding 100 nm to the pumping light wavelength) ofthe respective pumping lights are demultiplexd (more specifically, whenthe pumping wavelengths are 1430 nm and 1460 nm, wavelength lights near1530 nm and 1560 nm are demultiplexd). Each of the optical/electricalconverting means 19 serves to convert the received wavelength light intoan electrical signal, so that output voltage is varied with magnitude ofthe light receiving level. The LD control circuit 20 serves to changethe drive currents for the semiconductor lasers 3 in accordance with theoutput voltages from the optical/electrical converting means 19, and, bycalculating and processing the output voltage from theoptical/electrical converting means 19, the semiconductor lasers 3 arecontrolled so that light powers of the wavelength lights are ordered oraligned. That is to say, the output light power controlling means 4 actsto eliminate the wavelength dependency of the Raman gain thereby toflatten the gain.

In an arrangement shown in FIG. 5, a monitor signal detecting and LDcontrol signal generating circuit 15 comprising a power splitter 21,band pass filters 22, optical/electrical converting means 19 such asphoto-diodes and an LD control circuit 20 is connected to a monitorlight branching coupler 14. The power splitter 21 serves to branch theoutput light branched by the monitor light branching coupler 14 intolights having the same number as the pumping lights. The band passfilters 22 have different permeable central wavelengths, and, in thiscase, permit to pass lights near the maximum amplification wavelengths(each of which is a wavelength obtained by adding 100 nm to the pumpinglight wavelength) of the respective pumping lights (more specifically,when the pumping wavelengths are 1430 nm and 1460 nm, pass of wavelengthlights near 1530 nm and 1560 nm are permitted). Each of theoptical/electrical converting means 19 serves to convert the receivedwavelength light into an electrical signal, so that output voltage isvaried with magnitude of the light receiving level. The LD controlcircuit 20 serves to change the drive currents for the semiconductorlasers 3 in accordance with the output voltages from theoptical/electrical converting means 19, and, by calculating andprocessing the output voltage from the optical/electrical convertingmeans 19, the semiconductor lasers 3 are controlled so that light powersof the wavelength lights are ordered or aligned. That is to say, theoutput light power controlling means 4 acts to eliminate the wavelengthdependency of the Raman gain thereby to flatten the gain. Although FIGS.4 and 5 show arrangements for controlling the pumping means 1 bymonitoring the output light as shown in FIG. 27, as shown in FIG. 26, anarrangement for controlling the pumping means 1 by monitoring the inputlight can be utilized, or, as shown in FIG. 28, an arrangement forcontrolling the pumping means 1 by monitoring both the input light andthe output light can be utilized.

In the Raman amplifiers having the above-mentioned constructions, inplace of the fact the pumping lights are combined by the polarizationwave composing coupler 6, as shown in FIGS. 6A and 6B, a polarizationplane rotating means 7 for rotating polarization plane of the pumpinglight by 90 degrees may be provided, so that the plural pumping lightsgenerated by the pumping means 1 and pumping lights having polarizationplanes perpendicular to those of the former pumping lights aresimultaneously pumped in the amplifier fiber 2. FIG. 6A shows anarrangement in which a Faraday rotator 3 ₁ and a total reflecting mirror3 ₂ are provided at an end of the amplifier fiber 2 so that thepolarization plane of the pumping light propagated to the amplifierfiber 2 is rotated by 90 degrees to be returned to the amplifier fiber 2again. In FIG. 6, a means for picking up the optical signal propagatedto and Raman-amplified in the amplifier fiber 2 from the fiber 2 is notshown. FIG. 6B shows an arrangement in which a PBS 33 and a polarizationmaintaining fiber 34 are provided at an end of the amplifier fiber 2 sothat the polarization plane of the pumping light outputted from the endof the amplifier fiber 2 is rotated by 90 degrees by the polarizationmaintaining fiber 34 connected to twist its main axis by 90 degrees, tobe inputted to the end of the amplifier fiber 2 again through the PBS33.

(First Embodiment of Optical Repeater)

FIG. 7 shows a first embodiment of an optical repeater constructed byusing the Raman amplifier according to the present invention. In thisexample, the optical repeater is inserted into an optical fibertransmission line 8 to compensate loss in the optical fiber transmissionline 8. In this optical repeater, a rare earth doped fiber amplifier(referred to as “EDFA” hereinafter) 10 is connected to a rear stage ofthe Raman amplifier 9 as shown in FIG. 1, 2 or 3 so that the opticalsignal transmitted to the optical fiber transmission line 8 is inputtedto the Raman amplifier 9 to be amplified and then is inputted to theEDFA 10 to be further amplified and then is outputted to the opticalfiber transmission line 8. The gain of the repeater may be adjusted bythe Raman amplifier 9 or by the EDFA 10 or by both amplifiers so long asthe loss of the optical fiber transmission line 8 can be compensated intotal. Further, by properly combining a difference between thewavelength dependency of gain of the EDFA 10 and the wavelengthdependency of the Raman amplifier 9, the wavelength dependency of gainof the EDFA 10 can be reduced by the wavelength dependency of the Ramanamplifier 9.

(Second Embodiment of Optical Repeater)

FIG. 8 shows a second embodiment of an optical repeater constructed byusing the Raman amplifier according to the present invention. Accordingto this embodiment, in the optical repeater shown in FIG. 7, anadditional EDFA 10 is also provided at a front stage of the Ramanamplifier 9. Incidentally, the EDFA 10 may be provided only at the frontstage of the Raman amplifier 9.

(Third Embodiment of Optical Repeater)

FIG. 9 shows a third embodiment of an optical repeater constructed byusing the Raman amplifier according to the present invention. Accordingto this embodiment, there is provided a Raman amplifier 9 in which adispersion compensating fiber (DCF) is used as the amplifier fiber 2between two EDFAs 10. Between the Raman amplifier 9 and the EDFA 10 atthe rear stage thereof, there are provided a branching coupler 23 forbranching the output light from the Raman amplifier 9, and a monitorsignal detecting and LD control signal generating circuit 24 formonitoring the branched light and for controlling the gain of the Ramanamplifier 9. The monitor signal detecting and LD control signalgenerating circuit 24 is a control circuit capable of keeping the outputpower of the Raman amplifier 9 to a predetermined value. Incidentally,when the Raman amplifier 9 itself has the controlling means 4 as shownin FIG. 4 or FIG. 5, the power of the output light is controlled tobecome the predetermined value, and, at the same time, the power of thepumping light is controlled so that level deviation between pluraloutput signals become small.

In the optical repeater shown in FIG. 9, the output light level of theRaman amplifier 9, i.e., input level to the second EDFA 10 is alwayskept constant without being influenced by the loss of the DCF and theoutput level of the first EDFA 10. This ensures that, when the output ofthe repeater is defined, the gain of the second EDFA 10 is keptconstant. In this way, deterioration of gain flatness of the second EDFA10 due to fluctuation in loss of the DCF is avoided. Further, when thefirst EDFA 10 is controlled so that the gain becomes constant, thefluctuation of input to the repeater is compensated by the varying ofgain of the Raman amplifier 9. Namely, the adjustment is effected onlyon the basis of the gain of the Raman amplifier 9, with the result thatthe deterioration of gain flatness due to fluctuation in gain of theEDFA 10 can be avoided completely.

(Fourth Embodiment of Optical Repeater)

FIG. 10 shows an example that, in the embodiment shown in FIG. 9, acontrol means for adjusting the gain of the Raman amplifier 9 bymonitoring the light level is also provided between the first EDFA 10and the Raman amplifier 9. By using such control means, the pumpinglight can be controlled to keep the difference between the input andoutput levels of the Raman amplifier 9 constant, with the result thatonly the dispersion of the loss of the DCF can be compensated.

(Fifth Embodiment of Optical Repeater)

FIG. 11 shows an example that, in the above-mentioned embodiment, thegain flattening monitor mechanism is shifted to the output end of therepeater and is used as a monitor for flattening the gain of the entirerepeater. In this case, the first EDFA 10 and the second EDFA 10 mayperform the gain constant control or the output constant control. Thepowers of the pumping lights are independently controlled to reduce thelevel deviation between the output signals at the output of therepeater.

(Sixth Embodiment of Optical Repeater)

In an optical repeater according to a sixth embodiment of the presentinvention, a dispersion compensating fiber is used as the amplifierfiber 2 of the Raman amplifier shown in FIG. 1, 2 or 3 to compensatedispersion of wavelength of the optical fiber transmission line 8, sothat the losses in the optical fiber transmission line 8 and theamplifier fiber 2 are compensated partially or totally.

(Seventh Embodiment of Optical Repeater)

In the above-mentioned embodiment of the optical repeater, a Ramanamplifier 9 using an pumping means 1 shown in FIG. 41, 42, 43 or 44 maybe constituted.

(Eighth Embodiment of Optical Repeater)

As shown in FIGS. 29 to 32, WDM couplers 13 are inserted on the way ofthe amplifier fiber 2 of the Raman amplifier 9 so that residual pumpinglight from the pumping means 1 propagated to the amplifier fiber 2 isinputted to the optical fiber transmission line 8 through a WDM coupler27 provided in the optical fiber transmission line 8 at the input oroutput side of the Raman amplifier 9, thereby also generating Raman gainin the optical fiber transmission line 8. Incidentally, in FIGS. 29 to32, the reference numeral 26 denotes an isolator.

(Ninth Embodiment of Optical Repeater)

As shown in FIGS. 33 to 36, when the optical repeater comprises theRaman amplifier 9 and the EDFA 10, WDM couplers 13 are inserted on theway of the amplifier fiber 2 of the Raman amplifier 9 so that residualpumping light from the pumping means 1 propagated to the amplifier fiber2 is inputted to the EDFA 10 to be used as pumping light/auxiliarypumping light for the EDFA 10. Incidentally, in FIGS. 33 to 36, thereference numeral 26 denotes an isolator.

(First Embodiment of Raman Amplifying Method)

An embodiment of a Raman amplifying method according to the presentinvention will now be fully explained with reference to FIGS. 49 to 52.In this embodiment, a dispersion compensating fiber (DCF) having highnon-linearity is used in a Raman amplifying medium 50 shown in FIG. 49and pumping light generated from an pumping light source 51 is inputtedand transmitted to the Raman amplifying medium through a wave combiner52. In this case, as shown in FIG. 50, as the pumping light source 50, a4ch-WDMLD unit comprising four pumping light sources (semiconductorlasers), fiber Bragg gratings (FBG), polarization beam combiner (PBC)and a WDM is used. The semiconductor lasers shown in FIG. 50 generatepumping lights having different central wavelengths (more specifically,generate pumping lights having central wavelengths of 1435 nm, 1450 nm,1465 nm and 1480 nm, respectively). These pumping lights give Raman gainto an optical signal transmitted from the DCF, thereby amplifying theoptical signal. In this case, each of the pumping lights has gain peakat frequency shorter than its frequency by about 13 THz (i.e.,wavelength greater by about 100 nm).

When the optical signal having wavelength of 1500 nm to 1600 nm ispropagated to the DCF having a length of 6 km from one end thereof andthe pumping lights having wavelengths of 1400 nm, 1420 nm, 1440 nm, 1460nm, and 1480 nm are inputted to the DCF thereby to Raman-amplify theoptical signal, the input optical signal and an output optical signaloutputted from the other end of the DCF (Raman-amplified optical signal)were checked to evaluate total loss of the wavelength and DCF. Thefollowing Table 3 shows a relationship between the wavelength and DCFtotal loss. From this, it is apparent that there is wavelengthdependency:

TABLE 3 wavelength (nm) unit loss (dB/km) total loss (dB) 1400 6.7640.56 1420 3.28 19.68 1440 1.74 10.44 1460 1.16 6.96 1480 0.85 5.10 15000.69 4.14 1520 0.62 3.72 1540 0.57 3.42 1560 0.55 3.30 1580 0.54 3.241600 0.59 3.54

Here, when it is considered that effect total loss is given by addingthe loss of the semiconductor laser to the loss of the optical signalamplified by the semiconductor laser at the side of the longerwavelength greater by about 100 nm, a relationship between thewavelength and the effect total loss becomes as shown in the followingTable 4:

TABLE 4 wavelength (nm) effect total loss (dB) 1400 40.56 + 4.14 44.71420 19.68 + 3.72 23.4 1440 10.44 + 3.42 13.86 1460  6.96 + 3.30 10.261480  5.10 + 3.24 8.34

Since there is substantially no wavelength dependency of the Ramanamplification itself, when it is regarded that amplifying efficiency foreach wavelength is influenced by the effect total loss, by adding theeffect total loss to the outputs of the semiconductor lasers requiredfor desired amplifying properties, the wavelengths can beRaman-amplified substantially uniformly, and the wavelength dependencyof the gain can be eliminated. Thus, in this embodiment, the shorter thecentral wavelength of the pumping light the higher the light power.

(Second Embodiment of Raman Amplifying Method)

In a Raman amplifying method according to a second embodiment of thepresent invention, in order to Raman-amplify substantially uniformly theoptical signals having wavelengths of 1500 nm to 1600 nm transmitted bythe DCF, among two or more pumping lights incident on the DCF, lightpowers of pumping lights having wavelengths smaller than center (middle)between the shortest central wavelength and the longest centralwavelength are increased. More specifically, central wavelengths of thepumping lights generated by the pumping light source 51 shown in FIG. 50and incident on the DCF are selected to 1435 nm, 1450 nm, 1465 nm and1480 nm and the light powers thereof are selected as follows. That is tosay, the light powers of the pumping lights having the wavelengths of1435 nm and 1450 nm which are smaller than the center (1457 nm) betweenthe shortest central wavelength (1435 nm) and the longest centralwavelength (1480 nm) among four pumping lights incident on the DCF areincreased, as follows:

-   -   Light power of 1435 nm:563 mW    -   Light power of 1450 nm:311 mW    -   Light power of 1465 nm:122 mW    -   Light power of 1480 nm:244 mW

As a result, the gain profile of the optical signal having wavelength of1500 nm to 1600 nm (after Raman-amplified) transmitted by the DCFprovides gain of about 11 dB from 1540 nm to 1590 nm, with the resultthat the flatness becomes 1 dB, as shown in FIG. 51. That is to say, thewavelength lights transmitted by the DCF can be Raman-amplifiedsubstantially uniformly.

Incidentally, when the central wavelengths of the pumping lightsgenerated by the pumping light source 51 and incident on the DCF areselected to 1435 nm, 1450 nm, 1465 nm and 1480 nm and the light powersthereof are selected to 563 mW uniformly, the gain profile of theoptical signal having wavelength of 1500 nm to 1600 nm (afterRaman-amplified) transmitted by the DCF becomes as shown in FIG. 52.That is to say, although gain of about 24 dB can be obtained near thewavelength of 1580 nm, wide band gain flatness cannot be achieved (lossspectrum of the fiber is turned over).

(Third Embodiment of Raman Amplifying Method)

FIG. 53 shows a third embodiment of a Raman amplifying method accordingto the present invention. In the Raman amplifying method shown in FIG.53, pumping lights are combined by a wave combiner utilizing a principleof a Mach-Zehnder interferometer. The wavelengths of the pumping lightswhich can be combined becomes equidistant. In this embodiment, amongwavelengths which can be combined, several wavelengths are not used, andthe wavelength number at the short wavelength side of the pumping lightband is greater than the wavelength number at the long wavelength side.In this arrangement, when the power of all of the pumping lights havingvarious wavelengths are the same, the total power of the pumping lightsat the short wavelength side becomes greater than the total power of thepumping lights at the long wavelength side. This provides substantiallythe same effect, similar to the second embodiment, as the effectobtained by setting the power at the short wavelength side to be greaterthan the power at the long wavelength side under the condition that thepumping lights are arranged equidistantly. Accordingly, by setting asshown in FIG. 53, the gain profile can be flattened without creatinggreat difference between the powers of the pumping lights. This meansthat the total power of the pumping lights in which gain profile in apredetermined band can be flattened can be increased with determiningthe upper limit of the output power from one pumping light and thereforethat the gain of the amplifier can be increased.

(Sixth Embodiment of Raman Amplifier)

In an embodiment described hereinbelow, the shortest wavelengths of thepumping lights to be used are selected to 1420.8 nm (211 THz). Thereason is that the wavelength greater than 1530 nm (frequency belowabout 196 THz) which has been utilized in the present WDM systems isconsidered to be used as an amplification band. Accordingly, when awavelength greater than 1580 nm (frequency below about 190 THz) which isreferred to as an L-band is supposed as the amplification band, sincethe pumping band may be shifted by 6 THz, the shortest wavelength may beselected to 1462.4 nm (205 THz). Regarding the other amplificationbands, the shortest wavelengths can be determined in the similar manner.

FIG. 60 is a six embodiment of a Raman amplifier according to thepresent invention. Frequency of a first channel is 211 THz (wavelengthof 1420.8 nm) and frequencies of second to fifth channels are from 210THz (wavelength of 1427.6 nm) to 207 THz (wavelength of 1448.3 nm) andare arranged side by side with an interval of 1 THz. By combining thiswith an pumping light (frequency of 205 THz, wavelength of 1462. 4 nm)having a wavelength spaced apart from the fifth channel by 2 THz towardthe long wavelength side, the pumping means is formed. Accordingly, adistance or interval between the adjacent pumping wavelengths is withina range from 6 nm to 35 nm, and the number of pumping light sourceshaving center wavelengths at the short wavelength side with respect tothe center between the shortest center wavelength and the longest centerwavelength of each pumping light becomes greater than the number ofpumping light sources having center wavelengths at the long wavelengthside. The pumping light of each channel utilizes an pumping lightobtained by combining lights from semiconductor lasers of Fabry-Perottype (wavelengths of which are stabilized by fiber Bragg gratings (FBG))by means of a polarization beam combiner (PBC). Polarization wavecomposing is effected so as to increase the pumping power of eachwavelength and to reduce the polarization dependency of the Raman gain.When the pumping power obtained by output from the single laser isadequate, the laser output may be connected to the wavelength combinerafter depolarizing.

FIG. 61 shows Raman gain profiles when the pumping light sources shownin FIG. 60 are used. A curve A represents total gain, a curve Brepresents the sum of gains of the pumping lights of the first to fifthchannels, a curve C represents a gain of the sixth channel, and thinlines represent gains of pumping wavelengths of the first to fifthchannels. By multiplexing the pumping lights at the short wavelengthside with interval 1 THz, a smooth curve extending rightwardly anddownwardly can be formed, and, by adding this to a gain curve extendingrightwardly and upwardly due to the pumping lights at the longwavelength side, the total Raman gain is flattened. According to FIG.61, the gain obtained from the pumping lights at the short wavelengthside may be relatively small, but, since there are wavelength dependencyof loss of the pumping lights and Raman effect between the pumpinglights, the actual incident power at the short wavelength side must begreater than that at the long wavelength side. From FIG. 61, it can beseen that a projection of certain gain curve and a recess of anothergain curve cancel mutually by using the interval of 1 THz. FIG. 62 is anenlarged view of the total gain. A property in which the peak gain is 10dB, the gain band extends from about 196 THz (wavelength of 1529.6 nm)to about 193 THz (wavelength of 1553.3 nm) and the gain deviation isabout 0.1 dB is achieved.

FIG. 63 shows gain profiles when the wavelength of the sixth channel isa wavelength (frequency of 204.5 THz; wavelength of 1465.5 nm) spacedapart from the wavelength of the fifth channel by 2.5 THz toward thelong wavelength side in FIG. 60. Similar to FIG. 61, a curve Arepresents total gain, a curve B represents the sum of gains of thepumping lights of the first to fifth channels, a curve C represents again of the sixth channel, and thin lines represent gains of pumpingwavelengths of the first to fifth channels. Also in this case, by addinga gain curve extending rightwardly and downwardly due to the pumpinglights at the short wavelength side to a gain curve extendingrightwardly and upwardly due the pumping lights at the long wavelengthside, the total Raman gain is flattened. According to FIG. 63, the gainobtained from the pumping lights at the short wavelength side may berelatively small, but, since there are wavelength dependency of loss ofthe pumping lights and Raman effect between the pumping lights, theactual incident power at the short wavelength side must be greater thanthat at the long wavelength side. FIG. 64 is an enlarged view of thetotal gain. A property in which the peak gain is 10 dB, the gain bandextends from about 196 THz (wavelength of 1529.6 nm) to about 192 THz(wavelength of 1561.4 nm) and the gain deviation is about 0.1 dB isachieved. The gain band is wider than that in FIG. 62, and recess of thegain at a middle portion of the band is slightly greater. The reason isthat the interval or distance between the fifth channel and the sixthchannel becomes wider.

(Seventh Embodiment of Raman Amplifier)

FIG. 65 shows a seventh embodiment of a Raman amplifier according to thepresent invention. Frequency of a first channel is 211 THz (wavelengthof 1420.8 nm) and frequencies of second to eighth channels are from 210THz (wavelength of 1427.6 nm) to 204 THz (wavelength of 1469.6 nm) andare arranged side by side with an interval of 1 THz. The total number ofchannels is eight, and pumping light sources are constituted by usingsix wavelengths except for sixth and seventh channels. Accordingly, adistance or interval between the adjacent pumping wavelengths is withina range from 6 nm to 35 nm, and the number of pumping light sourceshaving center wavelengths at the short wavelength side with respect tothe center between the shortest center wavelength and the longest centerwavelength of each pumping light becomes greater than the number ofpumping light sources having center wavelengths at the long wavelengthside. The pumping lights of the channels are selected on demand, asdescribed in connection with the sixth embodiment. FIG. 66 shows Ramangain profiles when the pumping light sources shown in FIG. 65 are used.A curve A represents total gain, a curve B represents the sum of gainsof the pumping lights of the first to fifth channels, a curve Crepresents a gain of the eighth channel, and thin lines represent gainsof pumping wavelengths of the first to fifth channels. Also in thiscase, by adding a gain curve extending rightwardly and downwardly due tothe pumping lights at the short wavelength side to a gain curveextending rightwardly and upwardly due to the pumping lights at the longwavelength side, the total Raman gain is flattened. According to FIG.66, the gain obtained from the pumping lights at the short wavelengthside may be relatively small, but, since there are wavelength dependencyof loss of the pumping lights and Raman effect between the pumpinglights, the actual incident power at the short wavelength side must begreater than that at the long wavelength side. FIG. 67 is an enlargedview of the total gain. A property in which the peak gain is 10 dB, thegain band extends from about 196 THz (wavelength of 1529.6 nm) to about191 THz (wavelength of 1569.6 nm) and the gain deviation is about 0.1 dBis achieved. In comparison with FIG. 62 and FIG. 64, the gain bandbecomes further wider. The reason is that the longest pumping wavelengthis set to further longer.

(Eighth Embodiment of Raman Amplifier)

FIG. 68 shows an eighth embodiment of a Raman amplifier according to thepresent invention. Frequency of a first channel is 211 THz (wavelengthof 1420.8 nm) and frequencies of second to eighth channels are from 210THz (wavelength of 1427.6 nm) to 204 THz (wavelength of 1469.6 nm) andare arranged side by side with an interval of 1 THz. The total number ofchannels is eight, and pumping light sources are constituted by usingsix wavelengths except for fifth and sixth channels. Accordingly, adistance or interval between the adjacent pumping wavelengths is withina range from 6 nm to 35 nm, and the number of pumping light sourceshaving center wavelengths at the short wavelength side with respect tothe center between the shortest center wavelength and the longest centerwavelength of each pumping light becomes greater than the number ofpumping light sources having center wavelengths at the long wavelengthside. The pumping lights of the channels are selected on demand, asdescribed in connection with the sixth embodiment. FIG. 69 shows Ramangain profiles when the pumping light sources shown in FIG. 68 are used.A curve A represents total gain, a curve B represents the sum of gainsof the pumping lights of the first to fourth channels, a curve Crepresents the sum of gains of the seventh and eighth channels, and thinlines represent gains of pumping wavelengths. Also in this case, byadding a gain curve extending rightwardly and downwardly due to thepumping lights at the short wavelength side to a gain curve extendingrightwardly and upwardly due to the pumping lights at the longwavelength side, the total Raman gain is flattened. According to FIG.69, the gain obtained from the pumping lights at the short wavelengthside may be relatively small, but, since there are wavelength dependencyof loss of the pumping lights and Raman effect between the pumpinglights, the actual incident power at the short wavelength side must begreater than that at the long wavelength side. FIG. 70 is an enlargedview of the total gain. A property in which the peak gain is 10 dB, thegain band extends from about 196 THz (wavelength of 1529.6 nm) to about191 THz (wavelength of 1569.6 nm) and the gain deviation is about 0.1 dBis achieved. Here, it is noticeable that magnitude of the gains of thepumping wavelengths in the seventh embodiment differs from that in theeighth embodiment. Namely, there is a channel of about 8 dB at themaximum in FIG. 66; whereas, about 5 dB is maximum in FIG. 69. Thereason is that the gain at the long wavelength side shown by the curve Cis formed by the gain of one channel in the seventh embodiment, whereas,the gain at the long wavelength side is formed by the sum of the gainsof two channels in the eighth embodiment. This means that a maximumvalue of the pumping light power required for one wave can be reduced,which is very effective in the viewpoint of practical use.

FIGS. 71 to 74 show gain profiles when eleven channels are used amongthirteen channels arranged side by side at an interval of 1 THz from 211THz (wavelength of 1420.8nm) to 199 THz (wavelength of 1506.5 nm). FIG.71 shows the gain profiles obtained by an arrangement and by using thepumping lights other than 201 THz and 200 THz. Accordingly, a distanceor interval between the adjacent pumping wavelengths is within a rangefrom 6 nm to 35 nm, and the number of pumping light sources havingcenter wavelengths at the short wavelength side with respect to thecenter between the shortest center wavelength and the longest centerwavelength of each pumping light becomes greater than the number ofpumping light sources having center wavelengths at the long wavelengthside. A curve A represents total gain, a curve B represents the sum ofgains of the pumping lights of the first to tenth channels, a curve Crepresents the gain of the thirteenth channel, and thin lines representgains of pumping wavelengths of first to tenth channels. Also in thiscase, by adding a gain curve extending rightwardly and downwardly due tothe pumping lights at the short wavelength side to a gain curveextending rightwardly and upwardly due to the pumping lights at the longwavelength side, the total Raman gain is flattened. According to FIG.71, the gain obtained from the pumping lights at the short wavelengthside may be relatively small, but, since there are wavelength dependencyof loss of the pumping lights and Raman effect between the pumpinglights, the actual incident power at the short wavelength side must begreater than that at the long wavelength side. FIG. 72 is an enlargedview of the total gain. A property in which the peak gain is 10 dB, thegain band extends from about 196THz (wavelength of 1529.6 nm) to about186 THz (wavelength of 1611.8 nm) and the gain deviation is about 0.1 dBis achieved.

FIG. 73 shows the gain profiles obtained by an arrangement correspondingto an embodiment of a Raman amplifier, wherein when a certain pumpingwavelength is defined as a first channel, and second to n-th channelsare defined to be arranged with an interval of about 1 THz toward alonger wavelength side, the pumping light sources have the wavelengthscorresponding to all of the channels other than (n-2)-th and (n-3)-thchannels among the said first to n-th channels. The gain profiles ofFIG. 73 are obtained by using the pumping lights other than 202 THz and201 THz. Accordingly, a distance or interval between the adjacentpumping wavelengths is within a range from 6 nm to 35 nm, and the numberof pumping light sources having center wavelengths at the shortwavelength side with respect to the center between the shortest centerwavelength and the longest center wavelength of each pumping lightbecomes greater than the number of pumping light sources having centerwavelengths at the long wavelength side. A curve A represents totalgain, a curve B represents the sum of gains of the pumping lights of thefirst to ninth channels, a curve C represents the sum of the gains ofthe twelfth and thirteenth channels, and thin lines represent gains ofpumping wavelengths. Also in this case, by adding a gain curve extendingrightwardly and downwardly due to the pumping lights at the shortwavelength side to a gain curve extending rightwardly and upwardly dueto the pumping lights at the long wavelength side, the total Raman gainis flattened. According to FIG. 73, the gain obtained from the pumpinglights at the short wavelength side may be relatively small, but, sincethere are wavelength dependency of loss of the pumping lights and Ramaneffect between the pumping lights, the actual incident power at theshort wavelength side must be greater than that at the long wavelengthside. FIG. 74 is an enlarged view of the total gain. A property in whichthe peak gain is 10 dB, the gain band extends from about 196 THz(wavelength of 1529.6 nm) to about 186 THz (wavelength of 1611.8 nm) andthe gain deviation is about 0.1 dB is achieved. Further, as apparentfrom the comparison of FIG. 71 with FIG. 73, since the gain at the longwavelength side shown by the curve C is formed by the gain of onechannel in the seventh embodiment, whereas, the gain at the longwavelength side is formed by the sum of the gains of two channels in theeighth embodiment, a maximum value of the gain required for one wave issmaller in FIG. 73 than in FIG. 71. This means that a maximum value ofthe pumping light power required for one wave can be reduced, which isvery effective in the viewpoint of practical use.

(Ninth Embodiment of Raman Amplifier)

FIGS. 54A and 54B show Raman gain profiles when the intervals betweenthe pumping lights are 4.5 THz and 5 THz, respectively and DSF is usedas the amplifier fiber. As apparent from FIGS. 54A and 54B, as theinterval between the pumping lights is increased, the recess or valleyof the gain becomes deeper, and, thus, the gain deviation becomesgreater. Incidentally, in FIG. 54A, values shown in the following Table1 are used as frequencies (wavelengths) of the pumping lights, and, inFIG. 54B, values shown in the following Table 2 are used as frequencies(wavelengths) of the pumping lights. In such cases, the interval of 4.5THz between the pumping lights corresponds to 33 nm, and 5 THzcorresponds to 36.6 nm. That is to say, from these examples, it can beseen that, if the interval between the pumping lights is greater than 35nm, the gain flatness is worsened.

TABLE 1 Pumping frequency pumping wavelength wavelength interval THz nmnm 204.5 1466.0 33.0 200.0 1499.0

TABLE 2 Pumping frequency pumping wavelength wavelength interval THz nmnm 205.0 1462.4 36.6 200.0 1499.0

FIG. 55 shows gain profiles when the interval between the pumping lightsis 4.5 THz, and three wavelengths are used. From FIG. 55, when a thirdpumping wavelength is added, it can be seen that, if the intervalbetween the pumping lights is 4.5 THz, the recess of the gain becomesdeeper. FIG. 56 shows gain profiles when the intervals between thepumping lights are 2.5 THz and 4.5 THz and three wavelengths are used.In comparison with FIG. 55, the recess of the gain is shallower. Sincethe frequency interval of 2.5 THz used in this case corresponds to about18 nm between the wavelengths, also in this case, it can be said thatthe interval between the adjacent pumping wavelengths is within a rangefrom 6 nm to 35 nm.

Industrial Availability

In the Raman amplifier according to the present invention, by selectingthe wavelengths of the pumping light sources so that the intervalbetween the central wavelengths becomes greater than 6 nm and smallerthan 35 nm and the difference between the maximum central wavelength andthe minimum central wavelength becomes within 100 nm, there can beprovided an optical amplifier in which the wavelength dependency of gainis reduced to the extent that the gain flattening filter is not requiredand in which, if the gain is changed, the flatness can be maintained.Further, this amplifier can be applied to an optical repeater forcompensating loss of a transmission line and wavelength dispersion. In arepeater constituted by the combination of the amplifier and EDFA,fluctuation of the EDFA due to fluctuation of input of the repeaterand/or fluctuation in loss of DCF can be suppressed to avoiddeterioration of gain flatness and the repeater can be applied tovarious systems.

In the Raman amplifying method according to the present invention, sincethe shorter center wavelength of the pumping light among two or morepumping lights incident on the DCF the greater the power, or, since thepower of the pumping light at the short wavelength side with respect tothe center between the shortest central wavelength and the longestcentral wavelength among two or more pumping lights incident on the DCFis increased, in any cases, even when an optical fiber having highnon-linearity is used, wavelength multiplexing lights of about 1500 nmto about 1600 nm can be amplified with substantially the same gain. Inother words, by using the optical fiber having high non-linearity, therequired gain can be obtained even with a short optical fiber. Further,since the optical fiber can be shortened, a Raman amplifier which can beunitized can be provided.

As described in connection with the prior art, the wavelength combinerof Mach-Zehnder interferometer type is very useful for multiplexing thepumping lights efficiently. One of reasons why the pumping lights havingthe equal frequency interval are used is that such a wave combiner canbe used. FIGS. 57 to 59 show performance of Raman gain curves when theinterval between the pumping lights is equidistant. FIG. 57 shows acondition that adjustment is effected to bring the peak gain to 10 dBunder a condition that the gains of the pumping lights are the same.From FIG. 57, it can be seen that the smaller the interval between thepumping lights the smaller the unevenness of the gains. FIG. 58 shows anexample that the gains of the pumping lights are adjusted to flatten thegains. Also in this case, similar to FIG. 57, the interval between thepumping lights the smaller the unevenness of the gains. Further, it canbe seen that undulation of the gain curves in FIG. 57 determines themaximum gain deviation in FIG. 58. Thus, in order to keep the gaindeviation to about 0.1 dB, it is said that, although the interval of 2THz between the pumping lights is too great, 1 THz is adequate.

FIG. 59 shows performance when the interval between the pumping lightsis 1 THz and the multiplexing number is changed. As can be seen from a 1ch pumping gain curve, when a silica-based fiber is used, although asmooth curve without unevenness is presented at the short wavelengthside with respect to the gain peak, there are three noticeable localpeaks at the long wavelength side, and such unevenness becomes a factorfor determining limit of flattening. Such unevenness is reduced as thenumber of multiplexing is increased. For example, observing the 1 chpumping gain curve, although there is protrusion of about 1 dB near 187THz, as the number of multiplexing is increased, the protrusion isreduced. The reason is that, since the peak gains are set to be thesame, the gain per one pumping wavelength or the local peak itself isreduced as the number of pumping wavelength is increased, and that theslightly and equidistantly shifted unevenness having the sameconfiguration are added. That is to say, by adding the protrusion of thegain curve of a certain pumping wavelength to the recess of the gaincurve of another pumping wavelength, the unevenness is reduced totally.A value of “about 1 THz” as used herein is based on this principle andis based on the fact that, in the 1 ch pumping gain curve shown in FIG.59, frequency difference between the protrusion near 187 THz and theadjacent recess near 188 THz is about 1 THz. Accordingly, depending upona fiber used, the 1 ch pumping gain curve may be slightlydifferentiated, and, thus, the value of “about 1 THz” may be changed. Inany case, in order to reduce the gain deviation, it is required that theunevenness (projections and recesses) of the gain curves to be added orcombined to be cancelled.

Since the limit of the gain deviation is determined by undulation and/orunevenness of the gain curves to be overlapped, it is considered thatthe gain profile having good flatness and small gain deviation can beobtained by combining gain curves having less unevenness. Accordingly,this can be achieved by combining the gain curve obtained bymultiplexing the pumping lights at interval of 1 THz with the gain curveof the pumping light at the long wavelength side with respect to saidpumping wavelength. In this case, it is desirable that the peaks of twogain curves are moderately spaced apart from each other in the viewpointof the widening of the band.

The invention claimed is:
 1. A Raman amplifier comprising: a pump lightsource; and an amplifying optical fiber configured to transmit a signallight and amplify the signal light by a pumping light supplied from thepump light source, wherein the pump light source comprising: asemiconductor laser, an external resonator arranged to maintain apredetermined central wavelength of laser light emitted from saidsemiconductor laser notwithstanding fluctuation of a driving current ofsaid semiconductor laser, a laser light intensity control unitconfigured to control a gain profile of the Raman amplifier bycontrolling the driving current of said semiconductor laser, wherein thepredetermined central wavelength of the laser light being maintainedsubstantially constant by the external resonator.
 2. The Raman amplifieraccording to claim 1, wherein said pump light source further comprising:one or more additional semiconductor lasers, one or more additionalexternal resonators corresponding to the one or more additionalsemiconductor lasers respectively, each additional external resonatorbeing configured to maintain a central wavelength of a respectiveadditional semiconductor laser at a different wavelength than thecentral wavelength of said semiconductor laser, wherein a laser emissionbandwidth of each of said semiconductor lasers is as narrow as 3nm. 3.The Raman amplifier according to claim 1, wherein said pump light sourcefurther comprising: one or more additional semiconductor lasers, one ormore additional external resonators corresponding to the one or moreadditional semiconductor lasers respectively, each additional externalresonator being configured to maintain a central wavelength of arespective additional semiconductor laser at a different wavelength thanthe central wavelength of said semiconductor laser, the Raman amplifierfurther comprising: a monitor light branching coupler configured tobranch a part of the signal light transmitted through the amplifyingoptical fiber; a wavelength de-multiplexer configured to de-multiplexthe branched part of the signal light into a plurality of de-multiplexedsignal lights having different wavelengths; and a plurality of detectorsarranged to measure the intensity of each de-multiplexed signals,wherein the laser light intensity control unit controls driving currentsof the plurality of semiconductor lasers in response to the measuredintensities of the de-multiplexed signal lights by the plurality ofdetectors.
 4. A Raman amplifier according to claim 3, wherein each ofsaid different wavelengths corresponds to a wavelength of a maximum gaingenerated by pumping light of a corresponding pumping wavelength.
 5. ARaman amplifier according to claim 1, wherein said pump light sourcefurther comprising: one or more additional semiconductor lasers, one ormore additional external resonators configured to maintain a centralwavelength of a respective additional semiconductor laser at a differentwavelength than the central wavelength of said semiconductor laser; anamplifying optical fiber configured to transmit a signal light andamplify the signal light by a pumping light supplied from the pump lightsource; a monitor light branching coupler configured to branch a part ofthe signal light transmitted through the amplifying optical fiber; apower splitter configured to split the branched signal lights into aplurality of signal lights, the number of which corresponds to apredetermined number of central wavelengths of the pumping light; aplurality of detectors arranged to measure the intensities of the splitsignal lights; and a plurality of bandpass filters arranged between eachof the detectors and the power splitter respectively, each bandpassfilter having a different permeable central wavelength, wherein thelaser light intensity control unit controls the driving currents of thesemiconductor laser in response to the intensities of the split signallights measured by the plurality of detectors.
 6. The Raman amplifieraccording to claim 1, wherein the external resonator is a fiber Bragggrating.
 7. The Raman amplifier according to claim 6, further comprisinga polarization maintaining optical fiber provided as at least part of aconnecting optical fiber connecting the semiconductor laser and theexternal resonator, or as at least part of an output optical fiber ofthe external resonator, or as at least part of each of the connectingand output optical fibers, wherein the polarization maintaining fibersoutput linear-polarized light.
 8. The Raman amplifier according to claim1, wherein said pump light source further comprising: one or moreadditional semiconductor lasers, one or more additional externalresonators corresponding to the one or more additional semiconductorlasers respectively, each additional external resonator being configuredto maintain a central wavelength of a respective additionalsemiconductor laser at a different wavelength than the centralwavelength of said semiconductor laser, the Raman amplifier furthercomprising one or more polarization maintaining optical fiberscorresponding to the one or more additional semiconductor laser andprovided as at least part of a connecting optical fiber connecting thesemiconductor laser and the external resonator, or as at least part ofan output optical fiber of the external resonator, or as at least partof each of the connecting and output optical fibers, and thepolarization maintaining fibers output linear-polarized light.
 9. TheRaman amplifier according to claim 8, wherein the external resonator isa fiber Bragg grating.
 10. The Raman amplifier according to claim 1,further comprising a device configured to measure or monitor intensityof input or output signal light, and said control unit is furtherconfigured to control said laser current in order to control the gainprofile of the Raman amplifier.
 11. The Raman amplifier according toclaim 10, wherein said control unit is further configured to control again profile of the Raman amplifier based on the intensity of the inputor output signal light.
 12. The Raman amplifier according to claim 1,wherein said pump light source further comprising: one or moreadditional semiconductor lasers, one or more additional externalresonators corresponding to the one or more additional semiconductorlasers respectively, each additional external resonator being configuredto maintain a central wavelength of a respective additionalsemiconductor laser at a different wavelength than the centralwavelength of said semiconductor laser, the Raman amplifier furthercomprising a device configured to measure or monitor intensity of inputor output signal light, and said control unit is further configured tocontrol said laser current in order to control the gain profile of theRaman amplifier.
 13. The Raman amplifier according to claim 12, whereinsaid control unit is further configured to control a gain profile of theRaman amplifier based on the intensity of the input or output signallight.
 14. The Raman amplifier according to claim 1, wherein thesemiconductor laser is Fabry-Perot type.
 15. The Raman amplifieraccording to claim 14, wherein the central wavelength of the laser lightis within 1400 nm band, and a wavelength of the signal light within 1500nm band.
 16. A pump light source for a Raman amplifier, comprising: asemiconductor laser; an external resonator arranged to maintain apredetermined central wavelength of laser light emitted from saidsemiconductor laser notwithstanding fluctuation of a driving current ofsaid semiconductor laser; and a laser light intensity control unitconfigured to control a gain profile of the Raman amplifier bycontrolling the driving current of the semiconductor laser, wherein thepredetermined central wavelength of the laser light being maintainedsubstantially constant by the external resonator.
 17. The pump lightsource according to claim 16, further comprising one or more additionalsemiconductor lasers each having another external resonator configuredto maintain a central wavelength of laser light emitted from theadditional semiconductor at a different wavelength than the centralwavelength of laser light emitted from said semiconductor laser, whereinthe laser light intensity control unit is configured to control drivingcurrents of the plurality of semiconductor lasers in response tomeasured intensities of a plurality of signal lights.
 18. The pump lightsource according to claim 16, comprising: a semiconductor laser and oneor more additional semiconductor lasers, an external resonator arrangedto keep a predetermined central wavelength of the laser light emittedfrom said semiconductor laser notwithstanding fluctuation of a drivingcurrent of said semiconductor laser, said additional semiconductorlasers have other external resonators so as to keep their output lightat different central wavelengths compared to the wavelength of laserlight from said semiconductor laser, a laser light intensity controlconfigured to control a gain profile of the Raman amplifier bycontrolling the driving current of said semiconductor laser, thepredetermined central wavelength of the laser light being keptsubstantially constant by the external resonator, and the laser lightintensity control unit controls the driving currents of thesemiconductor laser in response to the intensities of the split signallights measured by the plurality of detectors.
 19. The pump light sourceaccording to claim 16, wherein an optical fiber connecting thesemiconductor laser and the external resonator, and an output opticalfiber of the external resonator include polarization maintaining fibers,and the polarization maintaining fibers output the linear-polarizedlight.
 20. The pump light source according to claim 16, furthercomprising one or more additional semiconductor lasers each havinganother external resonator configured to maintain a central wavelengthof laser light emitted from the additional semiconductor at a differentwavelength than the central wavelength of laser light emitted from saidsemiconductor laser, wherein an optical fiber connecting thesemiconductor laser and the external resonator, and an output opticalfiber of the external resonator include polarization maintaining fibers,and the polarization maintaining fibers output linear-polarized light.21. The pump light source according to claim 20, wherein the externalresonator is a fiber Bragg grating.
 22. The pump light source accordingto claim 16, wherein the external resonator is a fiber Bragg grating.23. The pump light source according to claim 16, further comprising adevice configured to measure or monitor intensity of input or outputsignal light, and said control unit is further configured to controlsaid laser current in order to control the gain profile of the Ramanamplifier.
 24. The pump light source according to claim 23, wherein saidcontrol unit is further configured to control a gain profile of theRaman amplifier based on the intensity of the input or output signallight.
 25. The pump light source according to claim 16, furthercomprising one or more additional semiconductor lasers each havinganother external resonator configured to maintain a central wavelengthof laser light emitted from the additional semiconductor at a differentwavelength than the central wavelength of laser light emitted from saidsemiconductor laser, and further comprising a device configured tomeasure or monitor intensity of input or output signal light, and saidcontrol unit is further configured to control said laser current inorder to control the gain profile of the Raman amplifier.
 26. The pumplight source according to claim 25, wherein said control unit is furtherconfigured to control a gain profile of the Raman amplifier based on theintensity of the input or output signal light.